In its simplest form a closed refrigeration system, such as is used in transport refrigeration equipment, serially includes a compressor, a discharge line, a condenser, an expansion device, an evaporator and a suction line connected to the compressor. In such a system, the mass flow, which is generally proportional to the suction, pressure, is indicative of the compressor power requirements. In normal operation, the suction gas is in the form of a low pressure superheated vapor and its state is dependent upon a number of factors such as ambient temperature, the temperature being maintained and whether the system has reached equilibrium or is still being pulled down. At startup, the suction gas may be at ambient temperature and even partially condensed in the evaporator and suction line. Under these conditions, and especially during conditions of pulldown, the mass flow of refrigerant drawn into the compressor and subsequently compressed is much greater than it would be in an equilibrium situation. This greater mass flow results in significantly increased power requirements. During operation, the mass flow rate will change with ambient temperature and other variations in the heat load on the space being refrigerated such as the exposure of a refrigerated cargo container to sun load.
Refrigeration systems that see a wide variation in air temperatures over the condenser and evaporator coils experience a correspondingly large variation in suction pressure and mass flow rate of the refrigerant, resulting in a wide variation in the power requirements of the compressor. A transport refrigeration system is an example of this. Cargo requirements can range from 55.degree. F. for bananas to -20.degree. F. for ice cream, and ambient temperatures can range from -20.degree. F. to 120.degree. F. There are, in addition, operating factors such as having to pull down the temperature of cargo loaded at a temperature greater than the desired setpoint, and size and weight limitations on the system components, which result in instances where the mass flow rate and pressure of the suction gas returning to the compressor, if not regulated, would overload the engine which drives the compressor.